1. Field of the Invention
The present invention relates in general to systems for interconnecting integrated circuits mounted on a circuit board and in particular to an interconnect system that provides a filter and impedance matching function.
2. Description of Related Art
Each node of an integrated circuit (IC) that is to communicate with external circuits is linked to a bond pad on the surface of the IC chip. In a packaged IC a bond wire typically connects the bond pad to a conductive leg extending from the package surrounding the IC chip. When the IC is mounted on a printed circuit board (PCB), the package leg is soldered to a microstrip PCB trace on the surface of the PCB. When bond pads of one or more other ICs mounted on the PCB are linked to the PCB trace, the bond pads, bond wires, package legs, and the PCB trace form an interconnect system for conveying signals between nodes of two or more ICs.
In high frequency applications the interconnect system can attenuate and distort signals. The conventional approach to reducing the amount of signal distortion and attenuation caused by the interconnect system has been to minimize the series inductance and shunt capacitance of the interconnect system. Much of the inductance comes from the bond wire and the package leg, and such inductance can be minimized by keeping the bond wire and package leg as short as possible. The capacitance of the bond pads can be reduced to some extent by minimizing the surface area of the bond pads. The capacitance of PCB traces can be reduced by appropriately choosing physical characteristics of the board including the size of the PCB traces, their spacing from ground planes and dielectric nature of the insulating material forming the circuit board. Vias, conductors passing vertically through a circuit board to interconnect PCB traces on various layers of the PCB, can be a source of capacitance at the PCB trace. Designers often avoid the use of vias in high frequency applications because vias can add substantial capacitance to the interconnect system. When vias are unavoidable, designers typically structure them so as minimize their capacitance. Minimizing the bond wire and package leg inductance and capacitances of the bond pad and PCB can help increase the bandwidth, flatten frequency response and reduce the signal distortion, but it is not possible to completely eliminate interconnect system inductance and capacitance. Thus some level of signal distortion and attenuation is inevitable when signal frequencies are sufficiently high.
What is needed is a way to substantially improve the frequency response of the interconnect system so as to reduce distortion and attenuation of high frequency signals.